Optical scanners with a Universal Serial Bus (USB) interface can be roughly classified into two categories; those that are completely powered from their USB interface (USB powered), and those that operate on power from an external power source. The major advantage of USB powered scanners is that of portability. USB powered scanners do not require external power and so can be used practically anywhere that a portable computer (which has a USB port) can be set up. Compact optical card scanners, for example, are typically USB powered as portability is an important feature since the card scanners are used in a wide variety of field applications where external power is not readily available.
As the power available from the USB is limited, card scanners that are USB powered usually incorporate small motors that supply limited torque, which results in lower performance in terms of scanning speed. The advance of imaging technology and storage has fuelled the need for higher performance card scanners which require more power to operate. One example is the duplex card scanner that is capable of scanning front and back sides of a card in a single pass. Another example is a card scanner that uses a high energy stepper motor to propel thick rigid cards through the scanning path.